Oxalis pes-caprae L. is a plant of the Oxalidaceae family, from which several compounds have been previously identified. Recently, we showed that an Oxalis pes-caprae L. extract inhibits the vasopressor effect of noradrenaline. In this work we aimed to explore the mechanisms involved in this effect. The results confirmed that the flavonoid fraction present in the extract inhibits noradrenaline-induced contractions and that this effect is concentration-dependent. Also, a parallel shift to the

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... ht in the noradrenaline concentration-response curve was observed, suggesting a decrease in efficacy and also in potency. Together these results support the assumption that the extract could exert a non-competitive antagonism on the α-adrenergic receptors. However, experiments in the presence of competitive antagonists for α-adrenergic receptor sub-types (i.e. prazosin, yohimbine and phentolamine) showed that the effect may not be directly mediated by α-adrenergic receptors. Thus, the interaction of this extract with the adrenergic system remains to be confirmed.